1. Field of the Invention
This invention relates to a process for making steel from solid metallic iron-bearing materials in a converter equipped with sub bath or submerged inlet tuyeres for supplying carbonaceous fuel and oxygen to the converter. In each converter cycle a quantity of solid iron-bearing material is loaded into the converter, and a quantity of molten iron-bearing material is then poured over the solid iron-bearing material. The molten iron-bearing hot metal charge originates from a preceding converter cycle. The total charge is melted down through the introduction of fuel and oxygen, and a portion of the hot metal charge is tapped into a storage vessel for a subsequent converter cycle.
2. Description of the Prior Art
A process for making steel from solid metallic iron-bearing materials is known from West German Unexamined Patent Application 29 39 859. In each converter cycle a portion, and preferably the major portion of the drawn off molten metal tap is delivered for subsequent processing, for example, to a continuous casting plant, while the remainder of the tap is stored separately and is used as a hot steel charge in a subsequent converter cycle wherein it is added to a charge of iron-bearing materials. Hence, this prior art steelmaking process involves a quasi continuous operation wherein a quantity of liquid pig iron or crude steel is required at the beginning of a series of converter cycles. In subsequent converter cycles the amount of liquid iron or steel used comes from the preceding converter cycle.
Therefore, in this known steelmaking process, steel can be produced from solid iron-bearing materials without requiring liquid pig iron and with greater energy efficiency in a relatively short tap-to-tap time and, hence, in a more economical fashion. The process is not limited to a pig iron production facility and enables steel to be made solely from solid iron-bearing materials, particularly scrap metal.
However, when practicing this known method, it has been found that in existing converters and with charges of pure scrap the full charge weight of the converter cannot always be attained because the required amount of scrap needed to reach the full charge weight cannot be introduced into the converter. As a rule, the scrap is so bulky that during charging, the converter becomes filled up before the desired amount of scrap, expressed as a weight, is reached. In other words, the charging of the scrap is limited by the volume of the converter, so that the full charge weight of the converter is not attained and the converter is not fully exploited.
It would be possible to introduce at first into the converter only some of the scrap that is to be melted in a converter cycle and charge the remainder later, e.g., in the form of scrap for cooling. However, the sequence of production steps would be interrupted by the subsequent post-charging, because the converter has to be tilted in the charging position.
Alternatively, converters could be utilized with a larger internal volume, that is, with a larger converter volume. Usually, the converter volume is defined by the available internal volume per ton of steel and is less than 1, typically 0.8, in most converters. However, there are also converters that have still smaller converter volumes. Since the scrap material, due to its bulkiness caused by a large percentage of included air gaps, has a specific weight of approximately 1, then one must realize that especially with a desired and more economical smaller repeat quantity of molten tap of, 20% to 50% of the total weight of the charge, the amount of scrap needed for the charging cannot be introduced into the converter. One must also bear in mind that the converter is typically filled in a tilting position, so that only 80% of the internal volume of the converter can be fully exploited. In a converter with a larger converter volume, for example, greater than 1, this problem does not arise. However, this type of larger converter, because of its larger internal surface, is less economical, since it leads to higher costs for refractory material. Furthermore, increasing the volume of the converter is no solution for adapting existing converters to the prior art process.
Another drawback of the prior art process is the fact that in each converter cycle one must finish-refine the tap because a portion of the tap, that is, a repeat portion, is always supplied for the subsequent processing. That is to say, the tap must have a desired steel quality and particularly a certain carbon content. According to the prior art process, the liquid tap is delivered to a storage vessel, such as a steel ladle. The amount of the repeat portion of steel tap provided for the repeat or reuse process cycle is carburized, preferably in the storage vessel. However, the carburization can also take place in the converter after the tapping of the amount of steel intended for the subsequent processing, but this prolongs the cycle time. As is well known, carburization causes the liquidus point to be lowered and thus no heat need be supplied to the storage vessel if there is a short time period until the next converter cycle.